This invention relates to carbon-containing coatings and to methods of and apparatus for the deposition of such coatings, and to articles bearing such coatings.
Carbon coatings are known per se. This invention is particularly concerned with carbon coatings having good hardness and wear resistance. Carbon coatings which have been produced in the past in order to give highly wear resistant surfaces have typically been diamond-like carbon coatings, i.e. carbon coatings which contain a significant amount of material with diamond, sp3, bonding.
This invention is based on research carried out with the aim of improving the tribological properties of carbon-containing coatings, and it is an object of this invention to provide a new carbon-containing coating with improved tribological properties and a method and apparatus which may be used to form such a carbon coating.
Accordingly, and in it first aspect, the present invention provides an article bearing a carbon-containing coating, wherein carbon-carbon bonding within the coating is mostly of the graphitic sp2 form.
It is indeed surprising that a carbon-containing coating which is graphitic in structure can in fact have tribological properties, such as greater hardness and wear-resistance, which are much improved over the properties of a carbon-containing coating which is of diamond-like structure.
Such a coating is preferably formed by sputtering, and in its second aspect, the invention provides a method of applying a coating to an article characterised in that such method comprises using a sputter ion plating system with at least one carbon target in which an ion current density of above 0.5 mA/cm2 is applied at the substrate to be coated in order to deposit a carbon coating layer.
This method provides a coating of much improved quality, in particular as regards its tribological properties. The coating layer contains sufficient carbon for there to be carbon-carbon (as opposed to carbide) bonds within it. When deposited in this way, it will be found that those carbon-carbon bonds are predominantly of the sp2 or graphitic type.
The invention also provides, in its third aspect, a method of applying a coating to an article characterised in that such method comprises using a sputter ion plating system with at least one carbon target in which an ion current density applied at the substrate to be coated is sufficiently high thereby to deposit a carbon coating layer having improved tribological properties and in which carbon-carbon bonds are predominantly of the graphitic sp2 form.
According to a fourth aspect of the invention, there is provided an article bearing a carbon-containing coating, wherein said coating comprises a succession of carbon and metal-containing layers and in which carbon-carbon bonding within the or each carbon layer is mostly of the graphitic sp2 form, and the invention extends to a method of applying a carbon coating to an article comprising using a sputter ion plating system with at least one carbon target and at least one metal target in order to deposit a carbon coating layer and a metal-containing coating layer.
The use of targets of different compositions, particularly when the substrate is suitably rotated, allows a considerable degree of control of the final coating composition and of different strata within it.
We have found that the presence of a said metal-containing layer can make a significant contribution to the coating properties. In some preferred embodiments of the invention, such a metal-containing layer is an underlayer deposited directly onto the substrate to be overcoated with carbon. Such a metal-containing underlayer is very effective in promoting adhesion between a carbon layer and a substrate.
Thus a method according to the invention preferably comprises using a metal target to form a metal-containing underlayer directly onto the substrate which is over-coated with the or a said carbon coating layer.
Such a metal-containing underlayer is preferably deposited to a thickness of between 50 and 200 nm.
The underlayer can comprise an initial stratum of metal followed by a second stratum consisting of the metal and carbon produced by co-deposition from metal and carbon targets. This stratum may be harder than the metal stratum and can improve the tribological behaviour of the subsequent carbon coating.
Alternatively, or in addition, there may be provided a one or more metal-containing layers each of which is an intermediate layer located between successive carbon layers. This promotes cohesiveness of the coating as a whole. We have found that when a uniform (graphitic) carbon coating is deposited on a substrate, once a certain thickness threshold is exceeded, the coating may have a tendency to spall and that this tendency increases with increasing thickness. We have also found, however, that this tendency is alleviated or even eliminated if thin carbon layers are interleaved with metal-containing layers. In order to avoid any risk of such spalling, a said carbon-containing layer is preferably formed to a thickness of up to 1 xcexcm.
Significant advantages are given by embodiments of the invention in which said coating is built up from a succession of alternating metal-containing and carbon coating layers respectively formed predominantly (that is, more than 50 atoms %) from metal and from carbon. By making use of this feature, the coating as a whole may be built up to any desired thickness, for example up to 10 xcexcm, preferably in the range 1 to 5 xcexcm, and without risk of spalling of the carbon, provided that the carbon layers have thicknesses below 1 xcexcm.
The use of a transition metal, that is a metal in one of Groups 3 to 10 (new IUPAC notation) of the Periodic Table as shown on page 1-15 of Handbook of Chemistry and Physics, 77th Edition 1996-1997 (CRC Press), preferably chromium or titanium, is found particularly effective.
A said coating is suitably formed by magnetron sputter ion plating, and most preferably by a technique known as closed field unbalanced magnetron sputter ion plating (CFUBMSIP). Such a technique can be used to produce coatings in accordance with this invention which have quite outstanding properties. Apparatus for use in such a technique is described later in this specification with reference to the accompanying diagrammatic drawing. Such apparatus is further described in UK Patent No. 2 258 343, and the full contents of that specification are incorporated herein by this reference.
Advantageously, at least two magnetrons are arranged to generate a magnetic field between them with field lines extending from one said magnetron to the or an other said magnetron, and said magnetrons, and field lines extending directly from one said magnetron to the or an other, form a barrier which tends to prevent the escape of electrons from within a plasma-containing working space within which the substrate is coated.
Such a method is particularly apt for achieving a high ion current density at in the region of the substrate being coated. The use of a CFUBMSIP method, in which at least partial closure of the field by such a barrier tends to prevent the escape of electrons from the working space, promotes ionisation in that space, and this in turn acts to promote the intensity of the ion bombardment the substrate will receive during deposition of the coating. As a result, a high ion current density (icd) can be achieved. In preferred embodiments of the invention, such a barrier to electron escape takes the form of a notional tube surrounding the substrate. The ends of such a tubular barrier may be open or closed.
Such a CFUBMSIP method of deposition is the preferred method of deposition although other methods giving high icd can also be used.
When sputter ion plating, the article being coated is preferably held at a bias voltage of from floating voltage to 250V negative. These are inclusive values, and the bias voltage applied may be DC, pulsed DC or RF. Preferably such bias is between 50V and 150V negative, and the best results have been attained when such bias is between 70 and 100 volts negative.
A very simple and preferred way of forming a said coating is to rotate the substrate while it is being coated. This assists in achieving an even coating on the substrate, and also has advantages in the deposition of a plural layer coating.
A particularly convenient way of achieving the optimum proportions of carbon and metal in a multi-layer coating is to use three carbon-donating targets and one metal-donating target for forming the coating. The substrate may be rotated in a working space between such targets and this is a highly advantageous way of building up a coating from a succession of alternating metal-containing and carbon strata.
The rotational speed of the substrate and the rates of deposition of the carbon and metal are suitably so controlled as to yield a period of at least 3 nm in the coating strata. If the period is too small, there is a risk that the carbon and metal will react and combine to form a metal carbide. While metal carbide may be tolerated in any said metal-containing layer, the replacement of carbon-carbon bonds by metal carbide bonds in a said carbon layer would detract from the quite remarkable properties afforded by this invention. It may be noted that it is the carbon layers having graphitic carbon-carbon bonds which provide these tribological properties; while such carbon layers may contain small proportions of metal (and indeed this may be difficult to avoid when using some deposition techniques) the presence of such metal is purely incidental, and the excellent wear-resistance and low friction properties do not depend on the carbon layers containing any metal. The metal-containing layers have a strengthening or adhesion promoting or anti-spalling effect, and they thus allow the coating to be built up to a greater thickness if this is desired. The incorporation of metal layers can also enable the coating to withstand higher loads.
Preferably, said coating is deposited under an atmosphere containing a noble gas. The use of a noble gas avoids contamination of the deposited coating by compounds of a chemically active gas. It is generally found most convenient to use argon. The argon gas pressure in the sputter system is not critical and can be between about 7xc3x9710xe2x88x922 Pa (5xc3x9710xe2x88x924 torr) and about 1 Pa (1xc3x9710xe2x88x922 torr). Preferably, said coating is deposited under an argon-containing atmosphere at a pressure of from 0.07 Pa to 0.6 Pa.
While it is desirable to avoid contamination of the coating by compounds of a chemically active gas, the invention does not exclude the possibility of deliberately modifying the coating composition by introducing such a gas into the atmosphere in the sputtering chamber.
Thus for example, the coatings which are formed can be modified by the inclusion of a hydro-carbon gas in the sputtering atmosphere during deposition as is provided in some embodiments of this invention. A feature of such a method may be that a mixture of carbon and metal is sputtered to deposit as a metal-containing layer on the substrate (the article being coated) which layer also contains additional carbon produced by breakdown of any hydrocarbon gas present in the working atmosphere and which deposits directly on the electrically biased substrate. Deposition of a said carbon layer may be by a combination of sputtering from the carbon target and breakdown of the hydrocarbon gas in the plasma.
Also, the coatings as described above can be modified by the addition of nitrogen to the working atmosphere during deposition. This can result in coatings modified by reaction of the nitrogen with the metal to form metal nitrides or with the carbon to from carbon-nitrogen compounds.
According to a further aspect of the invention we provide magnetron sputter ion plating apparatus having at least one metal-donating target and at least one carbon-donating target.
Such magnetron sputter ion plating apparatus preferably comprises at least two magnetrons adapted to generate a magnetic field between them with field lines extending from one said magnetron to the or an other said magnetron, whereby said magnetrons, and field lines extending directly from one said magnetron to the or an other, form a barrier which tends to prevent the escape of electrons from within a plasma-containing working space within which a substrate may be coated.
Preferably the arrangement is such as to permit the achievement of an ion current density of at least 0.5 mA/cm2.
The use of carbon and metal targets allows great flexibility in the choice of conditions under which the coating layers are deposited. For example such conditions may be varied to vary the composition of a said metal-containing underlayer as it is deposited, for example so that its composition varies from metal, adjacent the substrate surface, through metal carbide to (graphitic) carbon at the carbon layer.
The invention extends to a coating formed by a method as herein defined and to any article bearing such a coating, and also to:
An article bearing a carbon-containing coating, wherein said coating has a specific wear rate under wet conditions of less than 10xe2x88x9216 m3/Nm, and preferably less than 10xe2x88x9217 m3/Nm;
An article bearing a carbon-containing coating, wherein said coating has an adhesion critical load of at least 70 N;
An article bearing a carbon-containing coating, wherein said coating has a hardness of at least 1000 VHN;
An article bearing a carbon-containing coating, wherein said coating has a coefficient of friction (dry) of not more than 0.1;
Any such article wherein said coating is black and/or is electrically conducting.
A said carbon-containing layer should most preferably have a structure with one or more of the following properties:
an absence of crystallinity detectable by X-ray diffraction or by selected area diffraction in a transmission electron microscope;
a predominant carbon-carbon bonding within the layer of the sp2 form indicated by Raman spectroscopy;
predominantly graphitic (sp2) bonding with very small grain size;
The very high hardness which may be exhibited by the coatings indicates that there must be some cross bonding. The coating structure may contain some C60, and some sp3 bonding may be present.